Source: https://guthlabs.ai/docs/instruments/psychrometric-chart-tool/mechanism/

instrument mechanism / verified-with-binding-errata

# Psychrometric Chart Tool mechanism

The inspectable method record for Psychrometric Chart Tool, kept separate from implementation and qualification.

Status: **VERIFIED_WITH_ERRATA**Lifecycle: **verified-with-binding-errata**

## Model scope

The supplied formulas cover vapor-pressure conversion, humidity ratio, and enthalpy only. They do not uniquely specify the saturation correlation, wet-bulb equation, dew-point inversion, specific volume, valid ranges, or convergence behavior. The four vectors therefore use an explicit Magnus correlation and conventional moist-air equations; implementation should not be marked complete until those conventions are made part of the instrument specification.

- The saturation-pressure correlation is underspecified: Magnus has multiple coefficient sets and valid temperature ranges, while ASHRAE correlations are not interchangeable with Magnus.

- The wet-bulb algorithm is underspecified. A wet-bulb calculation requires an explicit psychrometric relation, such as W=Ws(Twb)−0.622·0.00066·(1+0.00115·Twb)·(Tdb−Twb), plus a stated pressure convention and convergence tolerance.

- Specific volume is claimed as an output but no formula is supplied. A standard ideal-gas moist-air formula is v=0.287042·(T+273.15)·(1+1.607858W)/P when T is in °C and P is in kPa.

- The claim that any two listed properties always determine a full state is too broad: pairs can be redundant, physically inconsistent, outside the correlation range, or insufficient without pressure and a defined thermodynamic model.

- The listed enthalpy formula is a conventional approximation for moist air with T in °C, h in kJ/kg dry air, and W in kg water/kg dry air; this basis and reference state must be displayed.

- Dew-point inversion and wet-bulb solving need explicit domain constraints, especially for supersaturated inputs, RH=0, negative absolute pressure, and temperatures outside the saturation-correlation range.

## Units and conventions

- Worked vectors use pws(T)=0.61094·exp(17.625T/(T+243.04)) kPa, with T in °C; this particular Magnus coefficient set must be fixed in the implementation.

- Pressure P is absolute pressure in kPa.

- Humidity ratio W is kg water/kg dry air; enthalpy h is kJ/kg dry air; specific volume v is m³/kg dry air.

- RH is a fraction in [0,1] in calculations; displayed percentages must be converted by dividing by 100.

- Wet-bulb calculations use W=Ws(Twb)−0.622·0.00066·(1+0.00115Twb)·(Tdb−Twb), solved by Newton iteration to |delta T|<0.000001 °C.

- The specific-volume checks use v=0.287042·(T+273.15)·(1+1.607858W)/P.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "dry_bulb_C": 25,
    "relative_humidity": 0.5,
    "pressure_kPa": 101.325,
    "saturation_correlation": "Magnus: 0.61094 exp(17.625T/(T+243.04))"
  },
  "expected_outputs": {
    "saturation_pressure_kPa": 3.16176,
    "vapor_pressure_kPa": 1.58088,
    "humidity_ratio_kg_per_kg_da": 0.0098595,
    "dew_point_C": 13.86,
    "wet_bulb_C": 17.9,
    "enthalpy_kJ_per_kg_da": 50.267,
    "specific_volume_m3_per_kg_da": 0.85707
  },
  "working": "pws(25)=0.61094·exp(17.625·25/(25+243.04))=3.16176 kPa. pv=0.5·3.16176=1.58088 kPa. W=0.622·1.58088/(101.325−1.58088)=0.983308/99.74412=0.00985950. Dew point: L=ln(1.58088/0.61094)=0.95070; Td=243.04L/(17.625−L)=13.8600 °C. Wet bulb uses Newton's method on f(Twb)=Ws(Twb)−0.622·0.00066·(1+0.00115Twb)(25−Twb)−0.00985950, tolerance 1e−6 °C: Twb(0)=15.0000 °C; Twb(1)=18.0200 °C; Twb(2)=17.9011 °C; Twb(3)=17.9000 °C; converged Twb=17.9000 °C. h=1.006·25+0.00985950·(2501+1.86·25)=25.1500+0.00985950·2547.50=50.2670 kJ/kg_da. v=0.287042·298.15·(1+1.607858·0.00985950)/101.325=0.857070 m³/kg_da."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The tool has a real undefined-result failure at P-p_v=0, and exactly zero relative humidity produces a non-finite dew point. The supplied specification also does not state the water/ice saturation branch or the unit conventions for temperature and humidity ratio, so different plausible interpretations produce different states.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/psychrometric-chart-tool/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/psychrometric-chart-tool/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/psychrometric-chart-tool/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche6/tranche6-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`